1. Field of the Invention
The present invention relates to an absolute type linear encoder of a so-called separate model, which includes a scale having a plurality of tracks, which include at least one of optical grids, electrode plates, magnetized magnetic bodies and electromagnetic induction coils each of which are continuously disposed at specified intervals along a measurement axis, and a detection head for detecting the relative position in relation to the scale, and is capable of adjusting the relative postures of the scale and the detection head when mounting the scale and the detection head on a machine that becomes an application object, and in particular, relates to an absolute type linear encoder capable of shortening the number of steps of adjusting the positional relationship between a scale and a detection head, and a method for adjusting the position thereof.
2. Description of the Related Art
As a linear encoder for feedback, which is used in a machine tool, etc., an absolute type linear encoder has generally been used, which is capable of detecting the absolute position when power is turned on.
In the absolute type linear encoder, there is a unit type (called an assembly type) having a mechanical structure by which the relative positions of a scale and a detection head are retained. Even when mounting such a unit type encoder in machine equipment such as a machine tool, this does not require any positional adjustment of the scale and the detection head of the absolute type linear encoder, and does not require adjustment of sift length with respect to offset, amplitude and phase of phase 2 sine-wave signal that is the output signal of the detection head and phase shifting between a plurality of tracks, which are produced by the change of the potion, after it is mounted because the relative positions of the scale and the detection head are retained.
Also, as the absolute type linear encoder, there is a separate type in which the scale and the detection head are separated from each other, differing from the unit type described above. This requires mechanical adjustment of the scale and the detection head with respect to the positional relationship and electric adjustment of sift length with respect to offset, amplitude and phase of phase 2 sine-wave signal detected by the detection head and phase shifting between a plurality of tracks thereafter when mounting the linear encoder in machine equipment such as a machine tool, etc.
Since, in the separate type, the scale and the detection head are separate from each other, several advantages are brought about, for example, the degree of freedom in assembly in machine equipment such as a machine tool, etc., is high, downsizing of the machine equipment can be achieved, there is no detection error due to the mechanical structure for retaining the relative positions thereof, and the design is not spoiled. However, it is necessary to combine two or more tracks (sets) of scales in the absolute type linear encoder. That is, in order to carry out highly accurate position detection, it is necessary to perform mechanical position adjustment having high accuracy with respect to a plurality of tracks and to reproduce the initial positional relationship (for example, the positional relationship in temporary assembly when shipping from a factory, for checking the precision).
Therefore, the present inventor has proposed a method for correcting the assembly position and adjusting data, which is shown in JP-A-2006-3307. JP-A-2006-3307 attempts to secure the initial positional accuracy (for example, the accuracy when shipping from a factory) by performing adjustment with respect to the obtained positional data based on the correction data held in an internal memory of a detection head and performing the correction of sift length with respect to offset, amplitude and phase of the detected signal after mechanically adjusting the detection head so as to obtain sufficient output from the detection head when assembling the detection head.
Generally, it is necessary to make the predetermined pitch (the cycle) of the least significant track, for example, the smallest pitch of the optical grid provided on the track, small in order to improve the resolution power and accuracy of an absolute type linear encoder, and it is necessary to lengthen the length of position detection of the most significant track in order to enlarge the detection distance of the absolute type linear encoder. These factors are achieved by increasing the cyclic ratio (track ratio) of the low-order track to the high-order track. However, if the track ratio is increased, the tolerance for mechanical adjustment between the scale and the detection head is reduced. At that time, since the length of the position detection of the most significant track is made longer, it becomes difficult to secure mechanical straightness over the entire scale length.
In JP-A-2006-3307, since the mechanical adjustment is carried out again if necessary even after electric signals are adjusted, the adjustment behavior is not easily converged in relation to the positional accuracy, wherein there is a fear that the positional adjustment is cumbersome. That is, there is a problem that the tolerance of mechanical adjustment between the scale and the detection head in the moving direction X of the scale cannot be increased.